Field of the Invention
The disclosure provides an improved endothermic hydrocarbon conversion process that comprises reacting a hydrocarbon with a multi-component catalyst bed, and regenerating the catalyst bed with air, where air flows used in the regeneration step compared to hydrocarbon flows are at relatively low air to hydrocarbon ratios and dehydrogenation can be operated at or near-atmospheric pressures.
Description of Related Art
Several endothermic hydrocarbon conversion processes are utilized in commercial operations. These processes include the Houdry cyclic fixed bed dehydrogenation process, the fluid bed paraffin dehydrogenation process, the fluid bed ethylbenzene dehydrogenation process, and fluid bed catalytic cracking process, among others. Because these processes are endothermic, heat must be consumed from the surroundings in order for the hydrocarbon conversion reaction to occur. In each of these processes, at least one reaction is promoted by contacting a hydrocarbon feed with a catalyst. Further, in each of these processes there is at least one reducing and/or oxidizing reaction that regenerates the catalyst. The heat needed for the endothermic reactions to occur is typically provided in part by combustion of coke and other undesirable side products that deposit on the catalyst during the conversion process. This combustion takes place during the regeneration process. Additional heat, however, is normally needed and is usually provided by hot air or steam that is fed into the catalyst bed from external sources between the hydrocarbon conversion cycles.
In a typical Houdry dehydrogenation process (e.g., the CATOFIN® process), an aliphatic hydrocarbon (e.g., propane) passes through a dehydrogenation catalyst bed and is dehydrogenated to a complementary olefin (e.g., propylene). The olefin is then flushed from the catalyst bed, the catalyst is regenerated and reduced, and the cycle is repeated.
This process can be run as an adiabatic, cyclic process. Each cycle includes a catalyst reduction step and a dehydrogenation step, and typically further includes a step to purge the remaining hydrocarbon from the reactor, and finally a regeneration step with air. Following this, the cycle begins again with the catalyst reduction step.
This dehydrogenation reaction is highly endothermic. Therefore, during the dehydrogenation step, the temperature near the inlet of the catalyst bed (where the aliphatic hydrocarbon initially enters the catalyst bed) can decrease by as much as 100° C. This decrease in temperature causes a decrease in hydrocarbon conversion. In addition, during the dehydrogenation step, it is common for coke to form and deposit on the catalyst, further reducing the activity of the catalyst.
In order to reheat the catalyst bed and remove coke that has deposited on the catalyst during the dehydrogenation step, the reactor is typically purged of hydrocarbon and then undergoes a regeneration step with air heated to temperatures of up to 700° C. Heat is provided to the bed by the hot air that passes through the bed and also by the combustion of the coke deposits on the catalyst. Reduction of the catalyst, with a reducing gas such as hydrogen, prior to dehydrogenation step also provides some additional heat.
During regeneration, the hot air flows from the top of the catalyst bed to the bottom, and the regeneration cycle is relatively short, so there is a tendency for the top of the bed to be hotter than the bottom of the bed. The lower temperature in the bottom of the bed does not allow full utilization of the catalyst and thus the yield is lower that what would be otherwise expected. Also, the coke distribution in the catalyst bed, which is not easily controlled, affects the amount of heat added at each location and the resulting catalyst bed temperature profile. These factors make control of the temperature profile in the bed difficult.
In the conventional HOUDRY CATOFIN® process, the reactor contains a physical mixture of a chromia/alumina catalyst and an inert component. The volume ratio between the inert component and the catalyst depends on a number of factors including the type of hydrocarbon feed being used in the dehydrogenation process. For example, for a propane feed the inert component can be equal to about 50% of the total catalyst volume, whereas for an isobutane feed the volume of the inert component can be as low as about 30% of the total catalyst bed volume.
The inert component is typically a granular, alpha-alumina material of similar particle size to the catalyst that is catalytically inactive with respect to dehydrogenation or side reactions such as cracking or coking, but that has a high density and high heat capacity, so it can be used to store additional heat in the bed. The additional heat is then used during the dehydrogenation step. However, the inert component is not capable of producing heat during any stage of the process.
Houdry catalyst bed temperatures may be controlled within a temperature range suitable for the reactions without requiring an extraneous heating or cooling fluid to be circulated through or around the reaction chamber by including within the catalyst bed an inert component capable of absorbing or storing up heat which can subsequently be released as desired or required. In commercial practice for fixed bed reactors, this is typically achieved by using a physical mixture of a dehydrogenation catalyst and a granular, alpha-alumina as the catalyst bed. Although the addition of the inert component provides a reversible heat sink for the process, and helps stabilize the overall temperature swings in the reactor, the inert component is not capable of providing extra heat for the process nor can it produce heat during any stage of the process. Hence, an external heat source is still required even with the combined use of the catalyst and the inert component.
Additional aspects of the technical background are described in U.S. Pat. Nos. 7,622,623 and 7,973,207, each of which is hereby incorporated by reference herein in its entirety.